Long Term Evolution (LTE) specifications support Component Carrier (CC) bandwidths up to 20 MHz (the maximum LTE Rel-8 carrier bandwidth). LTE operation with wider bandwidth than 20 MHz is possible, using multiple CCs, appearing as a number of LTE carriers to an LTE terminal. A straightforward way to obtain this would be by means of Carrier Aggregation (CA). The LTE standard supports up to five aggregated carriers, where each carrier is limited, according to the 3GPP specifications, to have one of six bandwidths, namely 6, 15, 25, 50, 75 or 100 RB (corresponding to 1.4, 3, 5, 10, 15 and 20 MHz respectively). The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink.
During initial access, an LTE CA-capable terminal behaves similarly to a terminal not capable of CA. Upon successful connection to the network, a terminal may, depending on its own capabilities and the network, be configured with additional CCs in the uplink (UL) and downlink (DL). This configuration is based on Resource Radio Control (RRC) signaling. Due to the heavy signaling and rather slow speed of RRC signaling, it is envisioned that a terminal may be configured with multiple CCs, even when not all of them are currently used.
In CA, the terminal (user equipment or UE) is configured with a primary CC (PCC), a primary cell (PCell) or a primary serving cell (PSC). The PCell is particularly important, e.g., due to control signaling on this cell and UE monitoring of the radio quality on the PCell. A CA-capable terminal can, as explained above, also be configured with additional carriers (or cells or serving cells) which are referred to as secondary CCs (SCC), secondary cells (SCell) or secondary serving cells (SSC). Note that these terms may be used interchangeably.
To further improve the performance of LTE systems, CA has been expanded to enable the use of LTE in an unlicensed spectrum. This operation is referred to as Licensed Assisted Access (LAA). As unlicensed spectrum may never match the qualities of licensed spectrum, the intention with LAA is to apply carrier aggregation and use a secondary carrier in an unlicensed band, while having a primary carrier in a licensed band. This will then ensure that the reliability associated with licensed carriers can be enjoyed for the primary carrier and only secondary carriers are used in unlicensed bands. However, operation of unlicensed carrier as standalone operation or CA with a primary carrier in an unlicensed band may also be employed. CA using licensed and unlicensed carriers is shown, for example, in FIG. 1.
According to 3GPP specifications under development, frame structure type 3 (FS3) is applicable to LAA secondary cell operation. In FS3, a radio frame is 10 milliseconds (ms) long and consists of 10 frames, each of 1 ms. Each subframe has two slots, each with a length of 0.5 ms. In Release 13 of the 3GPP specifications, all 10 subframes within a radio frame are for downlink transmissions. With LAA operation in uplink, the frame structure may introduce new configurations containing a mixture of downlink and uplink subframes in a radio frame.
In Dual Connectivity (DC) operation, the UE can be served by at least two nodes called master eNB (MeNB) and secondary eNB (SeNB). More generally, in multiple connectivity (multi-connectivity or MC) operation, the UE can be served by two or more nodes, such as an MeNB, SeNB1, SeNB2 and so on. The UE is configured with a primary component carrier (PCC) from both MeNB and SeNB. The primary cell (PCell) from MeNB and SeNB are called PCell and primary secondary cell (PSCell), respectively. The PCell and PSCell typically operate the UE independently. The UE is also configured with one or more secondary component carriers (SCCs) from each of MeNB and SeNB. The corresponding secondary serving cells served by MeNB and SeNB are called secondary cells (SCells). The UE in DC typically has separate transmission/reception for each of the connections with MeNB and SeNB. This allows the MeNB and SeNB to independently configure each UE with one or more procedures, such as radio link monitoring (RLM). DRX cycle, etc., on its PCell and PSCell, respectively.
In order to transmit in unlicensed spectrum, which is free and shared by everyone, some regulations have to be followed. Most regulatory bodies, including Europe's ETSI, require networks operating in unlicensed spectrum to use a Carrier Sense Multiple Access (CSMA) protocol. This means that transmitters are required to listen to the presence of carriers in the channel (or a time resource such as a symbol, time slot, frame, subframe, etc.) before occupying the channel and transmitting for a particular duration. This is performed by detecting energy on that particular channel for a channel sensing duration. Hence, this protocol is also known as Listen-Before-Talk (LBT) protocol.
Due to LBT, a transmission in an unlicensed band may be delayed until the medium becomes free again. In a case where there is no coordination between the transmitting nodes (which often is the case), the delay may appear random. More specifically, the transmitter node determines whether the channel is free or occupied by measuring the energy on the medium over a certain duration, i.e., an LBT measurement duration. If the channel is found to be free, the transmitter occupies the channel and can transmit during a channel occupancy time, which can extend over a certain number of time resources, such as between 4 ms and 10 ms. If the channel is found to be occupied, on the other hand, the transmitter node refrains from transmitting and waits until the channel becomes free.
In order to determine whether the channel is occupied or not during a particular LBT duration, a transmitter measures the energy detected during the LBT measurement duration and computes the corresponding power level. The power level is compared against a carrier sensing threshold, which may be referred to as an LBT threshold. If the power level is above the carrier sensing threshold, the channel is considered to be occupied. On the other hand, if the power level is below the threshold, then the channel is considered to be free.
The LBT procedure may also be called a channel-carrier-sense multiple-access (CSMA) scheme, a channel assessment scheme, a clear-channel assessment scheme, etc. The CSMA or LBT based operation is more generally referred to as contention-based operation. This contention-based operation is typically used for transmission on carriers of an unlicensed band. However, this mechanism may also be applied for operating on carriers belonging to licensed bands, for example, to reduce interference.
According to 3GPP TS 36.133 v13.2.0, the UE is required to update or adjust the uplink transmission timing based on some rules. The intent is to ensure that the uplink transmission timing does not drift in time, as that would result in difficulties in decoding the transmission at the eNB, or interference may even arise. The details of the uplink transmission timing adjustments are defined as follows (from 3GPP TS 36.133 v13.2.0):
7.1 UE Transmit Timing
7.1.1 Introduction                The UE shall have capability to follow the frame timing change of the connected eNodeB. The uplink frame transmission takes place (NTA+NTA offset)×Ts before the reception of the first detected path (in time) of the corresponding downlink frame from the reference cell. The UE shall be configured with a pTAG containing the PCell. The pTAG may also contain up to four SCells, if configured. The UE capable of supporting multiple timing advances may also be configured with one or two serving cells with uplink in one or two sTAG and pTAG.        The other downlink SCell(s), if configured, will be contained in either the pTAG or the sTAG(s). In pTAG, the UE shall use the PCell as the reference cell for deriving the UE transmit timing for cells in the pTAG. When the UE capable of supporting multiple timing advance is configured with one or two sTAG(s), the UE shall use an activated SCell from the sTAG for deriving the UE transmit timing for cells in the sTAG. UE initial transmit timing accuracy, maximum amount of timing change in one adjustment, minimum and maximum adjustment rate are defined in the following requirements. The requirements in clause 7 apply to all TAGs.        The UE capable of supporting dual connectivity shall be configured with one pTAG and may also be configured with one psTAG. The pTAG shall contain the PCell and may also contain one SCell, if configured. The psTAG shall contain the PSCell and may also contain one SCell, if configured. In pTAG, the UE shall use the PCell as the reference cell for deriving the UE transmit timing for pTAG, and in psTAG, the UE shall use the PSCell as the reference cell for deriving the UE transmit timing for psTAG. UE initial transmit timing accuracy, maximum amount of timing change in one adjustment, minimum and maximum adjustment rate are defined in the following requirements. The requirements in clause 7 apply to both TAGs.        
7.1.2 Requirements                The UE initial transmission timing error shall be less than or equal to ±Te where the timing error limit value Te is specified in Table 7.1.2-1. This requirement applies when it is the first transmission in a DRX cycle for PUCCH. PUSCH and SRS or it is the PRACH transmission. The reference point for the UE initial transmit timing control requirement shall be the downlink timing of the reference cell minus (NTA_Ref+NTA offset)×Ts. The downlink timing is defined as the time when the first detected path (in time) of the corresponding downlink frame is received from the reference cell. NTA_Ref for PRACH is defined as 0. (NTA_Ref+NTA offset) (in Ts units) for other channels is the difference between UE transmission timing and the downlink timing immediately after when the last timing advance in clause 7.3 was applied. NTA_Ref for other channels is not changed until next timing advance is received.        
TABLE 7.1.2-1Te Timing Error LimitDownlink Bandwidth (MHz)Te1.424 * TS≥312 * TSNote:TS is the basic timing unit defined in TS 36.211                When it is not the first transmission in a DRX cycle or there is no DRX cycle, and when it is the transmission for PUCCH, PUSCH and SRS transmission, the UE shall be capable of changing the transmission timing according to the received downlink frame of the reference cell except when the timing advance in clause 7.3 is applied. The UE is required to adjust its timing to within ±Te in a TAG when,                    changing the downlink SCell for deriving the UE transmit timing for cells in the sTAG configured with one or two uplinks,            in this TAG the transmission timing error between the UE and the reference timing exceeds ±Te,            configured with a pTAG and one or two sTAG, the transmission timing difference between TAGs does not exceed the maximum transmission timing difference (i.e., 32.47 us) after such adjustment.                        If the transmission timing difference after such adjustment is bigger than the maximum transmission timing difference (i.e., 32.47 us), the UE may stop adjustment in this TAG. The reference timing shall be (NTA_Ref+NTA offset)×Ts before the downlink timing of the reference cell. All adjustments made to the UE uplink timing under the above mentioned scenarios shall follow these rules:                    1) The maximum amount of the magnitude of the timing change in one adjustment shall be Tq seconds.            2) The minimum aggregate adjustment rate shall be 7*TS per second.            3) The maximum aggregate adjustment rate shall be Tq per 200 ms.            where the maximum autonomous time adjustment step Tq is specified in Table 7.1.2-2.                        
TABLE 7.1.2-2Tq Maximum Autonomous Time Adjustment StepDownlink Bandwidth (MHz)Tq1.417.5 * TS3 9.5 * TS5 5.5 * TS≥10 3.5 * TSNote:TS is the basic timing unit defined in TS 36.211